Drought-induced shifts in daily CO2 uptake patterns for leafy cacti

For cacti with persistent, relatively large leaves, most shoot CO₂ uptake under well-watered conditions occurs by the leaves using the C₃ pathway. For three species in the primitive subfamily Pereskioideae, droughts of 7 or 14 days decreased leaf daytime net CO₂ uptake by an average of 49 and 88%, respectively; these species always had a net CO₂ release at night by the leaves and both at night and during the day by the stems. For three leafy species in subfamily Opuntioideae, 7 and 14 days of drought reduced leaf daytime net CO₂ uptake by 90 and 100%, respectively. Although drought reduced the total CO₂ uptake over 24 h, the average percentage occurring at night by the leaves of these species increased from 5% under wet conditions to 71% after 7 days of drought to 99% after 14 days of drought. For two of the three species of Opuntioideae, 7 days of drought caused the small net CO₂ uptake by the sterns to shift from the daytime to the nighttime, while for the third species drought caused a reduction of its stem nocturnal net CO₂ uptake. Thus, shifts from predominantly daytime to predominantly nighttime net CO₂ uptake can be induced by drought for the leaves and the stems of leafy cacti in subfamily Opuntioideae, indicating a high degree of biochemical versatility.